Drill bit

ABSTRACT

The invention relates to a rotary drill bit for drilling a borehole in an earth formation, the drill bit comprising a first and a second disc cutter, each disc cutter being arranged to roll along the borehole bottom during rotation of the drill bit and thereby to cut respective first and second substantially circular, radially spaced cuts in the borehole bottom so that a body of rock material is defined between said cuts. The second disc cutter is arranged to cut into the bottom of the borehole at a selected rotational interval of the drill bit behind the first disc cutter and to shear-off said body of rock material in the direction of the first cut. Each disc cutter includes a set of mutually spaced cutting members arranged to create, during a turn of the drill bit, a corresponding set of cut sections of the cut created by the disc cutter, and wherein the cutting members are arranged so that the sets of cut sections created during subsequent drill bit turns are staggered relative to each other.

[0001] The present invention relates to a rotary drill bit for drillinga borehole in an earth formation. In the prior art various types ofdrill bits are applied, for example roller cone bits or jet cuttingbits. These drill bits are generally provided with abrasive cuttingelements which are made of a material having a high wear resistance suchas diamond or tungsten carbide. The cutting action of these cuttingelements mainly results from scraping of the elements along the boreholebottom. The progress of the drill bit in the borehole (i.e. the speed ofdrilling) depends on many factors such as the amount of wear of thecutting elements, the hardness of the rock and the weight on bit. As thecosts of drilling of a wellbore form a substantial part of the overallwellbore costs there is a continuous need to reduce the drilling time,viz. to increase the drilling speed.

[0002] WO 99/11900 discloses a rotary drill bit comprising a pluralityof disc cutter formed on roller cones and arranged to cut a plurality ofsubstantially circular, radially spaced cuts into the bottom of theborehole. The set of cutters of each roller cone is radially displacedrelative to the set of cutters of another roller cone, so that for eachpair of radially adjacent cutters a second one of the pair cuts into theborehole bottom at an angular interval behind the first one the pair. Abody of rock material is defined between the respective cuts, which bodyis sheared off by the second cutter in the direction of the cut createdby the first cutter.

[0003] It was found that the rate of penetration (ROP) into the earthformation of the known drill bit is relatively low in certain types ofrock formation, particularly in hard rock formations, due to a tendencyof shearing-off of the body of rock material in the direction of theupper end of the cut created by the first cutter. Accordingly there is aneed for an improved drill bit having an enhanced ROP when drilling insuch rock types.

[0004] In accordance with the invention there is provided a rotary drillbit for drilling a borehole in an earth formation, the drill bitcomprising a first and a second disc cutter, each disc cutter beingarranged to roll along the borehole bottom during rotation of the drillbit and thereby to cut respective first and second substantiallycircular, radially spaced cuts in the borehole bottom so that a body ofrock material is defined between said cuts, the second disc cutter beingarranged to cut into the bottom of the borehole at a selected rotationalinterval of the drill bit behind the first disc cutter and to shear-offsaid body of rock material in the direction of the first cut, whereineach disc cutter includes a set of mutually spaced cutting membersarranged to create, during a turn of the drill bit, a corresponding setof cut sections of the cut created by the disc cutter, and wherein thecutting members are arranged so that the sets of cut sections createdduring subsequent drill bit turns are staggered relative to each other.

[0005] By virtue of the feature that each disc cutter is formed of a setof discrete cutting members instead of the continuous disc cutter of theprior art, it is achieved that the instantaneous contact surface of thecutter with the borehole bottom is lower, leading to deeper penetrationof the drill bit into the rock formation for a given weight on bit. Thusthe cuts created by the cutters are deeper, and thereby shearing-off ofthe body of rock material tends to be more in the direction of the lowerend of the first cut than in the direction of the upper end thereof asin the prior art.

[0006] It will be appreciated that, since the disc cutter is no longercontinuous, the full circumferential cut has to be created in aplurality of drill bit turns. This is achieved by dimensioning the drillbit so that the cutting members cut during a subsequent drill bit turnin the rock material between the cut sections created by the cuttingmembers in a previous turn.

[0007] The invention will now be described by way of example withreference to the accompanying drawings in which

[0008]FIG. 1 schematically shows a bottom view of a drill bit accordingto the invention;

[0009]FIG. 2 schematically shows a partial side view of the drill bit ofFIG. 1;

[0010]FIG. 3 schematically shows a bottom view of a roller cone of thedrill bit of FIG. 1; and

[0011]FIG. 4 schematically shows a transverse cross section of aborehole bottom during various stages of drilling with the drill bit ofFIG. 1.

[0012] In the Figures, like reference numerals relate to like elements.

[0013] In FIG. 1 is shown a bottom view of a drill bit 1 having a bitbody 3 and three roller cones 4, 5, 6 arranged at 120° intervals withrespect to the axis of rotation 7 of the drill bit. The roller cones arerotatably connected to the bit body 3. Roller cone 4 is provided with aset of disc cutters 4 a, 4 b, 4 c, roller cone 5 is provided with a setof disc cutters 5 a, 5 b, 5 c, and roller cone 6 is provided with a setof disc cutters 6 a, 6 b, 6 c, each disc cutter 4 a, 4 b, 4 c, 5 a, 5 b,5 c, 6 a, 6 b, 6 c extending along the circumference of the respectiveroller cone in a plane substantially perpendicular to the axis ofrotation of the roller cone. Each disc cutter 4 a, 4 b, 4 c, 5 a, 5 b, 5c, 6 a, 6 b, 6 c is arranged to cut a substantially circular cut in theborehole bottom during rolling of the respective roller cone 4, 5, 6along the borehole bottom when the drill bit 1 is rotated.

[0014] Referring further to FIG. 2, there is shown a partial side viewof the drill bit 1 and roller cone 4. The roller cones 5, 6 are notshown for reason of simplicity, however the portions of cutters 5 b, 5c, 6 b, 6 c at the respective points of contact with the borehole bottomare shown in phantom. The radially outermost cutters 4 a, 5 a, 6 a arearranged so as to cut into the borehole bottom at equal radii relativeto the axis 7, i.e. these cutters are arranged at equal axial positionson the respective roller cones 4, 5, 6. The inner cutters 4 b, 4 c, 5 b,5 c, 6 b, 6 c are arranged on the respective roller cones 4, 5, 6 in anaxially (relative to the roller cone axis of rotation) staggeredrelationship. In other words, cutter 5 b cuts into the borehole bottomat a radius in between the radii at which cutters 4 a, 4 b cut into theborehole bottom, cutter 5 c cuts into the borehole bottom at a radius inbetween the radii at which cutters 4 b, 4 c cut into the boreholebottom, cutter 6 b cuts into the borehole bottom at a radius in betweenthe radii at which cutters 4 a, 5 b cut into the borehole bottom, andcutter 6 c cuts into the borehole bottom at a radius in between theradii at which cutters 4 b, 5 c cut into the borehole bottom.

[0015] Each disc cutter has two side surfaces 10, 12, whereby in FIG. 2only the respective side surfaces 10, 12 of cutter 4 a, 4 b, 4 c areshown for reason of simplicity. Each pair of side surfaces 10, 12 definea wedge-shaped cutting edge 14. Furthermore, each side surface 12extends perpendicular to the axis of rotation (indicated by referencenumeral 16 for roller cone 4) of the respective roller cone 4, 5, 6 sothat during drilling side surface 12 pushes against the rock in thedirection of the rotation axis 16. Side surface 20 extends parallel tothe axis of rotation 7 of the drill bit 1 at the point of contact withthe borehole bottom.

[0016] As shown in FIGS. 2 and 3, each disc cutter 4 a, 4 b, 4 c, 5 a, 5b, 5 c, 6 a, 6 b, 6 c is provided by a set of mutually spaced recesses20 whereby the remaining portions of the disc cutter define acorresponding set of mutually spaced cutting members 22. For each rollercone 4, 5, 6 the sets of cutting members 22 of adjacent cutters arearranged staggered relative to each other.

[0017] Each roller cone 4, 5, 6 is dimensioned so that the cuttingmembers 22 of each disc cutter of the roller cone create, during a turnof the drill bit 1, a set of cut sections (not shown) in the boreholebottom and that the sets of cut sections created during subsequent drillbit turns are rotationally displaced relative to each other. In otherwords, during a subsequent turn of the drill bit 1, the cutting members22 of the disc cutter cut into the borehole bottom at locations not cutbefore by the cutting members 22 before. This implies that the completecircular cut is created by the disc cutter in a plurality of drill bitturns.

[0018] In case the cutting members 22 are regularly spaced alongcircumference of the respective disc cutter, as is the case in theembodiment of the Figures, the desired dimensioning can be achieved ifit is avoided that during a subsequent drill bit turn the cuttingmembers 22 cut exactly in the same cuts as during previous turns. Oneway of achieving this is by avoiding the condition that L =i . T

[0019] in which

[0020] L =circumferential length of the cut,

[0021] i =integer number 1, 2, 3, . . . etc.,

[0022] T=pitch of the cutting members.

[0023] With T=π. d/n and L=π. D

[0024] d being the rolling diameter of the disc cutter,

[0025] n being number of cutting members of the disc cutter,

[0026] D being the diameter of the cut created by the cutter,

[0027] it follows that the drill bit is properly dimensioned if D/d =(i+f)/n.

[0028] Preferably 0.3<f<0.7. If, for example, f is about 0.5 the fullcut is created in about two drill bit turns.

[0029] Suitably D and d are selected so that 1.5<D/d<2.

[0030] In FIG. 3 is shown a bottom view of roller cone 4 with indicatedvalues of the angular extend of the cutting members (angular extend 32°)and the recesses (angular extend 28°). Also shown is the angular overlapof 2° of the cutting members of disc cutter 4 a with the cutting membersof disc cutter 4 b. A similar angular overlap exists between the cuttingmembers of the respective cutters 4 b, 4 c.

[0031] Furthermore the roller cones 4, 5, 6 are provided withconventional cutting inserts 26 at side surfaces 10 of the outermostcutters 4 a, 4 b, 4 c and at the inner parts of the roller cones.

[0032] During normal operation the drill bit 1 is rotated in theborehole, as a result of which the roller cones 4, 5, 6 roll and cutalong the borehole bottom. The cutting action is described hereinafterfor cutters 4 b, 5 b, 6 b only, as the action of the remaining cuttersis similar thereto.

[0033] Referring further to FIG. 4, the cutters 4 b, 5 b, 5 c createrespective adjacent cuts 32, 34, 36 in the borehole bottom. These cutsare indicated as straight lines, however in practice the cuts will havea shape corresponding more or less to the section of the cutterspenetrating the borehole bottom. Lines f, g, h, i, j indicate therespective envelopes of the lower ends of the disc cutters 4 b, 5 b, 5 cas a function of the rotation angle of the drill bit. Thus, startingfrom 0° bit rotation at borehole depth level f, line g indicates thelevel at 120° bit rotation, line h the level at 240° bit rotation, linei the level at 360° bit rotation and line j the level at 480° bitrotation.

[0034] At rotation angle 120° cutter 4 b has created cut 36 to level g.At rotation angle 240° cutter 5 b has created cut 34 to level h. A bodyof rock material 38 a is thereby defined between cut 36 at level h andcut 34 at level g, which body of rock material 38 a is sheared by sidesurface 12 of cutter 5 b along line s1 extending between the respectivecuts 36, 34.

[0035] At rotation angle 360° cutter 6 b has created cut 32 to level i,thereby defining a body of rock material 38 b between cut 34 at level gand cut 32 at level i, which body of rock material 38 b is sheared byside surface 12 of cutter 6 b along line s2 extending between therespective cuts 34, 32.

[0036] At rotation angle 480° cutter 5 b has created cut 34 to level j,thereby defining a body of rock material 38 c between cut 36 at level iand cut 34 at level j, which body of rock material 38 c is sheared offby side surface 12 of cutter 5 b along line s3 extending between therespective cuts 36, 34.

[0037] Similarly, after a further rotational angle of 120° a body ofrock material 38 d is defined between respective cuts 34 at level j and32 at a still deeper level, which body of rock material 38 d is shearedoff by side surface 12 of cutter 6 b along line s4 extending between therespective cuts 36, 34.

[0038] The outer cutters 4 a, 5 a, 6 a cut and shear off rock materiallocated between the cut (not shown) created by these cutters and cut 32created by cutter 5 b, in a manner similarly as described above. Thecutting action of cutters 4 c, 5 c, 6 c is similar to the cutting actionof respective cutters 4 b, 5 b, 6 b.

[0039] The discrete cutting sections created by the cutting members 22extend deeper into the borehole bottom than cuts which would have beencreated by continuous disc cutters (i.e. cutters without recesses) atequal weight on bit. Therefore the bodies of rock material, definedbetween adjacent cuts, have a larger size in the direction ofpenetration of the drill bit, so that the mere superficial cuttingaction during drilling in hard formations is avoided and thereby a moreefficient drilling process is achieved.

1. A rotary drill bit for drilling a borehole in an earth formation, thedrill bit comprising a first and a second disc cutter, each disc cutterbeing arranged to roll along the borehole bottom during rotation of thedrill bit and thereby to cut respective first and second substantiallycircular, radially spaced cuts in the borehole bottom so that a body ofrock material is defined between said cuts, the second disc cutter beingarranged to cut into the bottom of the borehole at a selected rotationalinterval of the drill bit behind the first disc cutter and to shear-offsaid body of rock material in the direction of the first cut, whereineach disc cutter includes a set of mutually spaced cutting membersarranged to create, during a turn of the drill bit, a corresponding setof cut sections of the cut created by the disc cutter, and wherein thecutting members are arranged so that the sets of cut sections createdduring subsequent drill bit turns are staggered relative to each other.2. The rotary drill bit of claim 1, wherein the cutting members aresubstantially regularly spaced along the cutter, and wherein the drillbit is dimensioned so that D/d=(i +f)/n in which D=diameter of the cutcreated by the cutter; n=number of cutting members of the cutter;d=rolling diameter of the cutter; i=integer number 1, 2, 3, . . . etc.;0<f<1.
 3. The drill bit of claim 2, wherein 0.3<f<0.7
 4. The drill bitof claim 2 or 3, wherein f is about 0.5.
 5. The drill bit of any one ofclaims 2-4, wherein 1.5<D/d<2.
 6. The drill bit of any one of claims1-5, wherein the drill bit includes a bit body and a plurality of rollercones rotatably connected to the bit body so as to substantially rollalong the borehole bottom during rotation of the bit in the borehole andbeing oriented in different radial orientations, a first one of theroller cones being provided with the first disc cutter and a second oneof the roller cones being provided with the second disc cutter.
 7. Thedrill bit of claim 6, wherein each roller cone is provided with aplurality of said disc cutters spaced along the axial direction of theroller cone, and wherein the sets of cutting members of adjacent disccutters of the roller cone are arranged in a staggered relationship. 8.The drill bit of any one of claims 1-7, wherein for each disc cutter,each cutting member of the disc cutter extends along a section angle αof the disc cutter and each spacing between adjacent cutting members ofthe disc cutter extends along a section angle β of the disc cutter, andwherein α=β+δ and 0°<δ<4°.
 9. The drill bit of claim 8, wherein 1°<δ<3°,and preferably δ is about 2°.
 10. The drill bit of any one of claims1-9, wherein each set of cutting members is defined by a set of mutuallyspaced recesses formed in the disc cutter to which the set of cuttingmembers pertains.
 11. The drill bit substantially as describedhereinbefore with reference to the drawings.